Field of the Invention
The invention relates to a vehicle steering system.
Discussion of Background
An electric power steering system, which is an example of a vehicle steering system, is a system that executes appropriate steering assist control in the following manner. A steering torque that is applied to a steering wheel (steering member) and a vehicle speed are detected, a steering torque signal and a vehicle speed signal are transmitted to an EPS electric motor control circuit, and the EPS electric motor control circuit supplies a driving current, which corresponds to the steering torque and the vehicle speed that are obtained from these signals, to a steering assist electric motor. A steering gear mechanism of this system includes a pinion shaft and a steering rack shaft. The steering assist electric motor is coupled to the pinion shaft. The steering rack shaft serves as a steered shaft that is in mesh with the pinion shaft and extends in the lateral direction of a vehicle. Note that there is a rack coaxial-type steering assist electric motor in which a steering assist electric motor is provided coaxially with a steering rack shaft. On the other hand, a steer-by-wire system (SBW) is a system in which a steering column mechanism on the steering side and a steering gear mechanism on the steered side are mechanically separated from each other, and which performs steering of a vehicle such that the steered angle of wheels is changed in accordance with the rotation of a steering member via an electric signal. A steering system electric motor is arranged in the steering gear mechanism, and a steered shaft that is coupled to the wheels is actuated by the steering system electric motor. An active front steering system (AFS) is a system that optimally controls the steered angle of wheels by using a variable gear ratio mechanism that is able to flexibly change the correlation between an operation amount of a steering wheel and the steered angle of the wheels. In each of the systems, knuckle arms are pivotably coupled respectively to a pair of end portions of the steering rack shaft via tie rods. A rightward force and a leftward force that are applied to the steering rack shaft each are referred to as “rack axial force”. When the steering member is steered or the steering system electric motor is rotated, a rack axial force is generated. As a result, the knuckle arms are pivoted on the basis of the generated rack axial force, and the steered wheels are steered.
For example, a steer-by-wire system described in Japanese Patent Application Publication No. 2003-2224 (JP 2003-2224 A) is a system in which a steering column mechanism on the steering side and a steering gear mechanism on the steered side are mechanically separated from each other, and which performs steering of a vehicle such that the steered angle of wheels is changed in accordance with the rotation of a steering member via an electric signal. A steering system electric motor is arranged in the steering gear mechanism, and a steered shaft that is coupled to the wheels is actuated by the steering system electric motor.
A variable transmission ratio steering system is a system that optimally controls the steered angle of wheels by using a variable transmission ratio mechanism that is able to change the correlation between an operation amount of a steering wheel and the steered angle of the wheels. In each of the systems, knuckle arms are pivotably coupled respectively to a pair of end portions of the steering rack shaft via tie rods. A rightward force and a leftward force that are applied to the steering rack shaft each are referred to as “rack axial force”. When the steering member is steered or the steering system electric motor is rotated, a rack axial force is generated. As a result, the knuckle arms are pivoted on the basis of the generated rack axial force, and the steered wheels are steered.
When the vehicle is stopped or when the vehicle is travelling at an extremely low speed, current that is supplied to the above-described steering assist electric motor or steering system electric motor is controlled in the same manner as that when the vehicle is travelling at normal speeds. That is, when the vehicle is stopped or when the vehicle is travelling at an extremely low speed, the steering torque is positively correlated with a rotation speed (steering speed) of the steering member. Therefore, when a steering operation is performed while the vehicle is stopped or the vehicle is travelling at an extremely low speed, a driving current corresponding to the steering speed flows through the steering assist electric motor or the steering system electric motor, and electric power is thus consumed.
FIG. 4 is a graph obtained by measuring the correlation, for each steering speed, between the steering angle and a driving current of a steering assist electric motor when a steering member is rotated from −720 degrees to +720 degrees while a vehicle is stopped in a conventional electric power steering system. The curve (1) shows data when the steering speed is 653 degrees per second (653°/sec). The curve (2) shows data when the steering speed is 332 degrees per second (332°/sec). The curve (3) shows data when the steering speed is 143 degrees per second (143°/sec). The curve (4) shows data when the steering speed is 27 degrees per second (27°/sec). The graph shows that, even if the rotation amount of the steering member is the same, the driving current becomes higher when the steering member is rotated quickly than when the steering member is rotated slowly.
In recent years, electrification of automobiles including electric vehicles and hybrid vehicles has been proceeding, and an electrical load on an in-vehicle power supply tends to increase. Therefore, a vehicle steering system is also required to consume less amounts of power.